induction heating of continuous carbon-ﬁbre-reinforced ...€¦ · induction heating of...

Induction heating of continuous carbon-fibre-reinforced thermoplastics

Abstract

This paper addresses the experimental investigation of induction heating of continuous carbon-fibre reinforced thermoplastics. Theinfluence of the process parameters electromagnetic frequency, generator power, distance between induction coil and laminate, coil geometryand laminate lay-up on the heating rate and the heat distribution have been investigated in stationary experiments. It was found that allinvestigated parameters have significant influence on the heating behaviour and that a quadratic dependence is dominating. Heat is onlygenerated when closed fibre loops exist, through which current can flow. The quality of the fibre junctions in a laminate, especially thecontact length, was found to be of major importance. Thus, for example laminates with unidirectional fibre reinforcement, which do notcontain fibre junctions, cannot be heated. Experimental evidence has shown that induction heating of carbon-fibre-reinforced thermoplasticsis based on Joule losses.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: E. Joining; Induction heating

1. The continuous induction welding process

Today’s market for high-performance thermoplasticcomposites is dominated by small and medium seriesproduction and prototyping. Therefore there is a strongneed for process technologies with minor capital investmentand high flexibility. For that reason a continuous inductionwelding process (CIW) for carbon-fibre-reinforced thermo-plastics (CFRT) has been developed at the Institut fuerVerbundwerkstoffe GmbH, Kaiserslautern (Germany),which is sketched in Fig. 1. The process meets the marketneeds very well because it requires only minor capitalinvestment and is especially designed for joining complexshaped parts [1].

With induction heating the transferable heat is for exam-ple 1500 times that of heat conduction (cf. Fig. 2) [2].

Some work has been performed on induction welding ofcomposites. In most cases metal susceptors have beenplaced at the welding interface in order to heat up thecomposite indirectly. Yarlagadda [3] reports on a novelconcept for metal mesh susceptors which were especiallydesigned to achieve a uniform in-plane heat distribution. Inthe EMAWELDw process welding is achieved by induc-tively heating a ferromagnetically filled thermoplastic

medium layer placed at the joint interface to the fusiontemperature of the abutting composite [4]. Benatar andGutowski [5] placed a nickel-coated graphite/J-polymerprepreg and two pure J-polymer sheets at the joint interfaceand heated and joined such composites.

Xiao [6] examined the heat affected zones caused bydifferent coil geometries and found in agreement with[7,8] that the heated composite area is a mirror image ofthe coil. The calculation of the heated zone showed goodagreement with the experiments.

Most of the earlier work was performed either withlaboratory devices and a stationary process was used orthere is no exact description of the used devices. Since astationary process limits the complexity of the parts to bejoined the present work focuses on a continuous process.

In the developed CIW the parts to be welded are movedwith a constant velocity under an induction coil and themagnetic field heats up the laminates. The welding pressureis applied by a cooled roller, positioned at a distancel fromthe induction coil, which is dependent on the laminate cool-ing behaviour.

The most important quality relevant feature of the CIW isthe temperature of the laminate during the four processphases (cf. Fig. 3).

1. Passing the induction coil the laminate surface and inter-facial temperature rises tou1, which represents the maxi-mum temperature to which the laminate is heated.u1 has

to be slightly higher than the welding temperature (stocktemperature), because the laminate temperaturedecreases until the pressure roller applies the weldingpressure. However,u1 has to be lower than the maximumpermissible temperature of the matrix material to avoidthermal degradation.

2. When the laminate reaches the pressure roller it hascooled down to the actual welding temperatureu2 byheat convection to the surrounding air and heat conduc-tion to the workpiece fixture as well as to the adjacentlaminate regions. This temperature has to be high enoughto enable the interdiffusion of the macromolecules andmolecular chains in order to effect the weld.

3. After the laminate has passed the roller the temperatureu3 has to be low enough to prevent delamination anddeconsolidation of the laminate. In order to cool downthe laminate sufficiently the roller is water-cooled.

4. Behind the roller an increase in the laminate temperatureto a peak temperatureu4 can be observed, which iscaused by the residual heat stored in the laminate. Toprevent defects in the laminate,u4 has to be lower thanthe recrystallisation temperature of the matrix material.This can be achieved by an additional cooling bycompressed air. Indeed, the laminate temperature should

be held at the recrystallisation temperature for a certainperiod of time to complete the crystallisation of thematrix material. Otherwise, if the laminate is used at atemperature above the recrystallisation temperature,recrystallisation and shrinkage of the matrix materialmight occur, which in turn can lead to warpage or lami-nate defects like matrix cracks or delamination.

This paper focuses on the inductive heat generation inphase I.

2. Induction heating principle

When an electrically conductive, non-magnetic materialis exposed to an alternating magnetic field, eddy currents areinduced and the material is heated due to resistive losses ofthe eddy currents. In magnetic materials, hysteretic lossesoccur which lead to an additional heat generation [2].Carbon-fibre reinforced thermoplastics can be inductionheated without any additional material since the carbon-fibres are electrically conductive. Glass-fibre reinforced

R. Rudolf et al. / Composites: Part A 31 (2000) 1191–12021192

Fig. 1. Schematic representation of the continuous induction weldingprocess.

Fig. 2. Transferable heat with different heating mechanisms [2].

Fig. 3. Typical temperature-time-curve of the continuous induction weldingprocess.

thermoplastics on the other hand, can only be induction heatedby means of an additional electrically conductive susceptormaterial which is placed between the parts to be joined.

The principle of induction heating of carbon-fibre rein-forced thermoplastics was investigated by Miller et al. [7]and Fink [8]. However, both found contrary reasons for theheat generation. Miller et al. found that eddy currents areinduced in the carbon fibres, and that electrical currenttransfers between the fibres and fibre layers since they arein contact or close proximity. The heat generation is causedby Joule losses in the fibres. However, the existence ofconductive loops is required, so that the unidirectionalcarbon-fibre laminates cannot be induction heated.

In a subsequent work to [7] a two-dimensional model wasdeveloped for induction heating and a Macintosh based soft-ware programme was developed [9]. With this, the eddycurrent distribution, the field strength distribution and thetemperature distribution of the laminate surface is made

possible. The agreement between the measured and thecalculated temperature distribution was very good.However, the programme is not commercially available.

Fink, on the other hand, claimed that even for laminateswith fibre volume fraction of 60% there is not contact orclose proximity between the fibres. Therefore, the heatgeneration is caused by dielectric losses in the polymerbetween the fibres. The laminate is modelled as a capacitorwith several capacitive layers represented by the fibrelayers, which are insulated from one another by polymerregions.

Fink also calculated the inductive heat generation andtemperature distribution in CFRT. Despite the contraryapproaches for the heat generation mechanisms both Finkand Miller reported good agreement between the experi-ment and the theory. However, the heat generation mechan-isms are not clear yet. Therefore, the investigationspresented in this paper focus on the experimental determi-nation of the parameter influences on the induction heatingprocess. To obtain basic quantitative information for subse-quent process modelling a stationary process was used forthe experiments.

3. Equipment

The experiments were performed with a 650–1000 kHz,5.2 kW induction generator. While the voltage is holdconstant at 220 V, the machine self-tunes the frequencyand coil current for each particular coil and load used.The power is pulsed as depicted in Fig. 4 and the powersetting is proportional to the coil current. The maximuminductance that can be fixed to the generator is 25mH,resulting in a maximum coil area of 500 mm2.

R. Rudolf et al. / Composites: Part A 31 (2000) 1191–1202 1193

Fig. 4. Power output of the induction generator.

Fig. 5. Experimental set-up for the induction heating investigations.

The magnetic field emitted by the coil could not bemeasured due to the difficulty of measuring fields at highfrequencies.

4. Experimental

The aim of this investigation is to predetermine theprocess parameters of the CIW with the help of a minimumnumber of preliminary experiments and a simple experi-mental set-up. Therefore, stationary experiments were firstcarried out to investigate the influence of the process para-meters electromagnetic frequency, generator power,distance between the induction coil and workpiece, coilgeometry and laminate lay-up. Fig. 5 shows the experimen-tal set-up. The laminate was placed between woodenclamps, such that free convection to both sides was possibleand heat transfer to the fixture could be neglected. Thelaminate temperature was measured by means of an infraredcamera.

5. Materials

The investigations of the process parameter influenceswere carried out with carbon-fibre fabric reinforced poly-phenylensulfide (CF-PPS) (5-harness satin; fibre volumefraction: 46%; thickness: 2 mm). The influences of the lami-nate structure were investigated with carbon-fibre rein-

forced polyamide 66 (CF-PA66) (fibre volume fraction:50%).

6. Results

6.1. Influence of the electromagnetic frequency

The alternating magnetic fieldB induces a voltageuind ina conductive fibre loop which is defined as

uind � 4·B·A� 2p·f·m·H·A �1�

wheref is the frequency and4 the angular frequency of themagnetic field,m the permeability of the workpiece mate-rial, H the magnetic field intensity andA is the area enclosedby the conductive fibre loop [10].

In an ohmic resistor (carbon fibre in this study) theinduced current is dissipated as Joule lossesP [11], given by

P� u2ind

Rf� 4p2·f2·m2·H2·A2

Rf�2�

where Rf is the electrical resistance of the carbon fibres.Hence, the inductive heat generationQind is proportionalto the frequency squared

Qind , f 2 �3�


Fig. 6. Influence of the frequency on the heating rate (triangular coil, power 20%, distance coil–laminate 5 mm).

Experiments with different induction generators showed,that while a carbon-fibre fabric reinforced thermoplasticcannot be heated beyond 508C at 20 kHz; nevertheless itcan be heated to 3008C at 1 MHz in less than 5 s and at26 MHz in less than 1 s.

Fig. 6 shows that the influence of the frequency on theheating rate is very strong even in the small range of 750–1100 kHz. These heating experiments were carried out witha triangular coil, which was placed at a distance of 5 mm ontop of the laminate, and a generator power of 20%. The time


Fig. 7. Influence of the generator power on the heating time.

Fig. 8. Influence of the distance between induction coil and laminate on the heating time.


Fig. 9. Dependence of the heating time on the distance between coil and laminate.

Fig. 10. Dependence of the heating time on the generator power.

was measured when the hottest point on the laminate surfacereached 3008C.

6.2. Influence of the generator power and the distancebetween coil and laminate

Heating experiments with different power levels anddistances between coil and laminate were performed inorder to investigate the significance of these process para-meters for he heating phase of the CIW. Differently frominduction heating of metals there was no coupling effectmeasured between coil and workpiece with CFRT so thatthe coil current is constant independent of the distancebetween the coil and the laminate. This can be explainedwith the low magnetic permeability of CFRT that is close tothat of air. In Fig. 7 the heating time is depicted as a function

of the temperature for different power levels, a constantdistance of 4 mm between coil and laminate and a frequencyof 1 MHz. The heating time is growing exponentially withthe temperature.

Fig. 8 shows the heating time versus the temperature fordifferent distances between coil and laminate and a constantpower of 40%. Again the heating time is growing exponen-tially with the temperature. This corresponds with the theoryof ohmic heat generation

Q � Qmax 2 a·et=t �4�wheret is the time,Q the temperature,Qmax is the equili-brium temperature, (Qmax 2 a) represents the temperature att � 0 and t represents the time at whichQ � 0:63 umax:

Notice that in Fig. 8 the inverse function is depicted.


Fig. 11. Selection of the investigated coil geometries and schematic representation of heat-affected areas (left: double-D; middle: circular pancake with squaretube; right: clip).

Fig. 12. Maximum reachable temperatures for different power levels and distances between coil and laminate.

While the heating time is increasing quadratically withthe distance between the coil and the laminate (cf. Fig. 9), itis decreasing quadratically with the power (cf. Fig. 10).

In another set of experiments the influence of the powerand the distance between the coil and the laminate on themagnetic field intensity was investigated, which in turninfluences the induced current and the maximum achievabletemperature.

A double-D coil (cf. Fig. 11a) was placed on top of a CF-PPS laminate at distance of 10, 15 and 20 mm, respectively.The laminate was heated using power levels of 10 and 20%,respectively, and the laminate surface temperature wasmeasured by means of an infrared camera. The inducedcurrent determines the maximum temperature to which alaminate can be heated. Analogous to Eqs. (2) and (3) itcan be written as

P� uind·iind � Rf ·i2ind �5�

) Q , i2ind �6�wherei ind represents the induced eddy currents.

Fig. 12 shows that the maximum temperature and, there-fore, the current intensity are growing with increasingpower. This means that the pulsation of the power (cf.Fig. 4) causes a summation of the heating effects of thesingle current pulses induced in the laminate. Furthermore,the current intensity is decreasing with the distance betweenthe coil and the laminate. This is due to the decrease of themagnetic field intensityH with the distance from the coil,which is given by

H � i4p

·Z 1

u~r u2· d~l × ~r

u~r u

� �� 7�

wherei is the coil current, dl a section of the coil length, andr is the distance between the coil and some pointP [10]. Thedegree of the field intensity decrease with the distance fromthe coil is dependent upon the coil geometry. The fieldintensity of a circular coil, for example, decreases with 1/r3, whereas for a linear coil of infinite length it decreaseswith 1/r [12].

6.3. Influences of the induction coil geometry and position

For the CIW the heating rate is not solely importantbut also the temperature distribution in the plane of theworkpiece and in through-thickness direction, which ismainly influenced by the induction coil geometry and, there-fore, the field intensity. The aim is a uniform temperature


Fig. 13. Orientation of the coil to the laminate.

Fig. 14. Influence of the coil geometry on the heat generation (N: number of windings).

distribution. Heating experiments with several inductioncoil geometries were performed and the heating rate andtemperature distribution was measured by means of aninfrared camera. The coils were produced such thatthe electromagnetic frequency was 1 MHz for all coilgeometries.

This distance between coil and laminate was 5 and10 mm, respectively, and the power was 35%. Since the5-harness satin weave is an asymmetrical fabric, it wasalso investigated if the coil orientation has an influence onthe heating rate and temperature distribution. For thatpurpose the experiments were carried out with the coilaxis oriented in 0, 45 and 908 direction to the laminate, asdepicted in Fig. 13.

Fig. 11 shows a selection of the investigated coil geome-tries and the shape of the heat-affected area which wasmeasured with an infrared camera. It can be seen that theheating pattern is a mirror image of the shape of the induc-tion coil. The double-D coil and the clip coil effect an ellip-tical heating zone with a uniform temperature distribution inthe plane of the workpiece. The heating zones of all otherinvestigated coil geometries show no closed structure buthave a cold spot in their middle or at the edge. In the CIWthis cold spot will disappear because of the movement of theworkpiece but it leads to a reduced feed velocity.

Fig. 14 shows the period of time until the hottest point onthe laminate surface has reached 3008C. Since the frequencyand the power were constant for all experiments the differ-ence of the heating times is caused by the different electro-magnetic field intensities. It can be seen that a decrease ofthe distance between coil and laminate from 10 to 5 mmleads to a decrease of the heating time of 300–400%. Thisis caused by the decrease of the electromagnetic field inten-sity with the distance from the induction coil (refer to Eq.(7)). Exception represents the circular pancake coils and thelittle triangular coil. The experiments were stopped after120 s since the laminate did not reach 3008C. These resultsshow that for some coil geometries the decrease of the fieldintensity with the distance has bigger influences on the heat-ing behaviour than for others, and that the distance betweencoil and laminate is a very sensitive parameter of the induc-tion heating process. The biggest heating rate (about 1008C/s) was achieved with the oval pancake coil.

The orientation of the coil has no measurable effect on theheating behaviour. The differences in the heating times canbe traced back to measuring inaccuracies.

6.4. Temperature distribution over the laminate thickness

A temperature gradient over the laminate thickness canbe caused by two phenomena.

1. The skin depth is smaller than the laminate thickness,which causes a skin effect.

2. The field intensity decrease over the thickness is strongenough to cause a temperature gradient.

The skin depthd is defined as

d ��

r

p·f·m

r�in m� �8�

wherer is the electrical resistivity of the heated material (inV m), f the electromagnetic frequency (in Hz) andm is themagnetic permeability (in H/m) [12].

For non-ferromagnetic materials like CFRT the magneticpermeabilitym is equal to that of air�m � m0 � 1:256×1026 H=m�:

The measurement of the resistivity of the investigatedCF-PPS gave

r' � 3 V m and rk � 5 × 1024 V m

wherer' is the resistivity in through-thickness directionperpendicular to the fibre-axis andr k is the resistivity inthe plane of the workpiece parallel to the fibre-axis.The set-up for the resistivity measurements is depicted inFig. 15.

Insertion ofm , f andr k in Eq. (8) yields a skin depth ofd � 12:6 mm for the investigated CF-PPS. Hence atemperature gradient due to a skin effect is impossible.

Stationary heating experiments with different powerlevels and distances between induction coil and laminatewere carried out in order to investigate the influence ofthe field intensity decrease on the through-thickness heating.The laminate was oriented vertically to achieve equalconvective conditions on both surfaces and the surfacetemperature was measured by means of an infrared camera.


Fig. 15. Experimental set-up for the resistivity measurements.

The investigations showed no temperature gradient inthrough-thickness direction of a 2-mm thick CF-PPSlaminate.

6.5. Influence of the laminate structure

Heating experiments were carried out with different lami-nate lay-ups in order to determine the influence of the textilestructure on the heating rate and the temperature distribu-tion. The following carbon-fibre textiles were investigatedseparately and in combination with each other:

• plain weave (pw);• 5-harness satin weave (s);

• fleece (f);• unidirectional tape (ud).

The distance between the induction coil and the laminatewas 5 mm and the power was 20% for all experiments. Thetendency of the heating results was equal for all investigatedcoil geometries, only the absolute heating rates varied.Therefore, the results are shown in Fig. 16 exemplary fora circular pancake coil (cf. Fig. 11b). The influence of thecoil orientation (0, 908) on the heating rate can be neglected,since the differences in the heating rates are small. Noticethat the laminate with the fleece and the unidirectional fibreorientation could not be heated. This indicates that there areno electrical-conductive paths in such laminates throughwhich current can flow. However, other fleece materialscould be heated to temperatures of up to 2508C. Thus,further investigation into the heating mechanisms of fleecematerials has to be performed.

The plain weave is heated faster than the satin weave,which indicates that the plain weave contains more electri-cal-conductive loops. This can be explained by the structureof the fabrics (cf. Fig. 17). The total number of fibre junc-tions is equal for both fabrics, but in the plain weave thereare more fibre junctions, at which the fibres have a bigcurvature (see arrows in Fig. 17). This indicates that thecurrent flow is enhanced at these junctions resulting in abigger heating rate. Microscopic studies of laminate cross-sections showed that there is no closer proximity of thefibres in 0 and 908 direction at these junctions. However


Fig. 16. Influence of the laminate lay-up on the heating rate for a pancake coil (pw, plain weave; s, satin weave; f, fleece; ud, unidirectional; p, polymerinterlayer).

Fig. 17. Schematic representation of the investigated fabrics.

the contact length is larger compared to the fibre junctionswith a straight fibre orientation, as schematically outlined inFig. 18 for a 5-harness satin weave. Indeed, the total contactlength in a laminate with a plain weave reinforcement is15% higher than that of a laminate with a 5-harness satinweave reinforcement. This correlates well with the experi-ments (cf. Fig. 16).

The laminates having polymer interlayers with a differentthickness (e.g. pw-p-pw, cf. Fig. 16) show a dependence ofthe heating rate on the thickness of this polymer layer. Thethicker the interlayer, the longer is the heating time. Fink [8]found the same for cross-ply laminates and explained it withthe “capacitive layer model”. The dielectric heat generationdecreases when the dielectric layer between the capacitorplates increases.

The differing behaviour of the satin weave reinforcedlaminates is not clear and might be caused by differencesin the processing conditions.

The study of micrographs of laminate cross-sectionsrevealed that there is contact or at least a close proximityof less than 5–10mm between the crossing fibre bundlesallowing the current to flow. Therefore, in the case of theinvestigated laminates with fabric reinforcement, theheat generation seems to be dominated by Joule lossesin the fibres. The heat generation is increasing with thecurrent density, which in turn is dependent on the fibre–fibre contact at the fibre junctions. This parameter is notchanged when a polymer layer is added between thefabrics. Therefore, the increasing heating times canonly be caused by the additional mass that has to beheated.

6.6. Mechanism of induction heating of carbon-fibre fabric-reinforced thermoplastics

Induction heating experiments were carried out with apolyamide 12 composite containing one layer of plainweave on one hand and one layer of pure plain weave with-out matrix polymer on the other hand. Both samples wereinductively heated using the same process parameters andthe temperature was monitored with an infrared camera. Theexperiments showed that the maximum achievable tempera-ture was equal for both samples, which means that thematrix polymer does not influence the heating mechanism,and dielectric heating, as proposed by Fink [8], can beexcluded. This is supported by the fact that a pure polya-mide 12 film could not be heated, too, at the investigatedelectromagnetic frequency. Hence, induction heating offabric reinforced thermoplastics is based on current flowat the fibre junctions and Joule losses in the carbon fibres,which supports the theory of Miller et al. [7].

7. Conclusions

The influence of the process parameters electromagneticfrequency, generator power, distance between induction coiland laminate, coil geometry and laminate lay-up on theheating rate and the heat distribution have been investigatedin stationary experiments.

In accordance with Miller et al. and Fink [3,4] it wasfound that heat is only generated when closed fibre loopsexist. Thus, for example laminates with unidirectional fibre


Fig. 18. Fibre contact-length of the fibre junction types and total number of junction types for an area of 7× 7 threads (cross sections are composed of fibrebundles).

reinforcement, which do not contain fibre junctions, could notbe heated. Moreover, the texture of the reinforcement wasfound to be of major importance, namely the quality of thefibre junctions. A model is presented for the correlation of thetotal contact length between the fibres in 0 and 908 direction ina laminate and the achievable heating rate. The inductionheating mechanism was found to be based on Joule losses inthe carbon fibres since current is flowing in the laminate.

The strongest influence on the heating behaviour has themagnetic field intensity distribution, which in turn is influ-enced by the power, the induction coil geometry and thedistance between coil and laminate. It was found that theheating time increases quadratically with the distancebetween induction coil and laminate and decreases quadra-tically with the generator power. The maximum temperatureto which a laminate can be heated is dependent on the powerand the distance between induction coil and laminate. Theheat distribution in through-thickness direction was found tobe uniform, whereas the heat-affected area in the plane ofthe workpiece is a mirror image of the magnetic field. Thecoil orientation had no influence on the heating rate and themaximum achievable temperature even for induction heat-ing of asymmetrical reinforcements like satin weaves.

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[9] Lin W, Miller AK, Buneman O. Predictive capabilities of an induc-tion heating model for complex-shape graphite fiber/polymer matrixcomposites. Proceedings of 24th International SAMPE ElectronicsConference, 1992. p. 606–20.

[10] Frohne H. Elektrische und magnetische felder. Stuttgart: TeubnerVerlag, 1994.

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